Figure 1

Before Darwin (1859) , similarities and differences in morphology, the ‘queen of the sciences’ in the early Enlightenment years, were explained in pre-evolutionary terms. Geoffroy St.-Hilaire stressed the ‘Unity of Type’ found in similar animals such as mollusks and vertebrates; Cuvier stressed the ‘Conditions of Existence’ by which otherwise similar animals differed by virtue of ecological specializations. Darwin’s simple definition of evolution as ‘descent with modification’ proposed that most similarities can be traced to common descent, whereas most differences in broadly similar (related) animals result from divergent adaptation.